putc               38 user/printf.c      putc(fd, buf[i]);
putc               45 user/printf.c    putc(fd, '0');
putc               46 user/printf.c    putc(fd, 'x');
putc               48 user/printf.c      putc(fd, digits[x >> (sizeof(uint64) * 8 - 4)]);
putc               65 user/printf.c          putc(fd, c0);
putc              100 user/printf.c          putc(fd, va_arg(ap, uint32));
putc              105 user/printf.c            putc(fd, *s);
putc              107 user/printf.c          putc(fd, '%');
putc              110 user/printf.c          putc(fd, '%');
putc              111 user/printf.c          putc(fd, c0);